Titanium is widely known for its use as a metal, but the primary use of titanium is in the form of titanium dioxide (TiO2). TiO2 is used as a white pigment in paints, plastics and paper.
There are two commercial processes for the production of TiO2 pigment namely, the sulphate process and the chloride process. The sulphate process comprises dissolving the feedstock in the form of titanium ore (such as ilmenite) or titania slag in concentrated sulphuric acid. The liquid containing dissolved TiO2 is then subjected to hydrolysis to yield solid TiO2. In the chloride process, the feedstock in the form of titanium ore or titania slag is fluidized at a high temperature (for example 950-1200° C.) in chlorine gas to produce a gas including TiCl4. The TiCl4 is then selectively condensed from the gas and is mixed with oxygen at high temperatures to yield TiO2.
A sulphate process plant is easier to operate and monitor than a chloride process plant, and is capable of using feedstock with a relatively low TiO2 content.
However, capital costs of a modern sulphate process plant can be higher than that of a chloride process plant of the same pigment capacity. Furthermore there is a higher volume of waste products to be treated and disposed of due to the use of more impure feedstock and the fact that the sulphate used In the process cannot be easily recovered and recycled. Accordingly the chloride process is a more favourable process and is growing in popularity.
One of the requirements of the chloride process is that the particle size specification of the feedstock must be suitable for use In a fluidized bed reactor. This particle size is typically in the range of 106 μm to 850 μm±5% (by mass).
As mentioned above the feedstock is either titanium ore or titania slag. In recent developments titania slags are upgraded and the upgraded slag is then used as a feedstock.
Titania slag is generally produced by reduction of ilmenite ores in an electric arc furnace to form pig iron and titania rich slag. The titania rich slag so formed is cast in a molten state into ladles whereby solid blocks are produced ranging from a few tons to as much as forty tons. After cooling, these blocks are crushed and then milled down to a particle size in the range of 106 to 850 μm rendering it suitable for use in the chloride process. It has been found that during this milling process some of the slag is reduced to a particle size below 106 μm rendering it too fine for use in the chloride process.
Agglomeration of titania slags is known in the art. U.S. Pat. No. 2,723,903 discloses a process wherein a densified, briquetted mixture of titaniferous material (including Sorel type slag containing approximately 70% by mass TiO2), coking coal (some non-coking coal can also be added) and a carbonaceous binding agent is chlorinated in a shaft furnace. Briquettes were produced with dimensions of 50.8 mm by 50.8 mm by 31.8 mm. The briquettes as exemplified had an estimated TiO2 and FeO)/C mass ratio of 0.7 to 1.3.
U.S. Pat. No. 2,805,120 discloses a similar process as in U.S. Pat. No. 2,723,903.
U.S. Pat. Nos. 4,117,076 and 4,187,117 disclose pelletised Sorel slag with bituminous coking coal and a suitable binder. The pellets were of a suitable size to be used in a fluidized bed reactor. These pellets again had a high carbon content and the pellets exemplified typically had a (TiO2 and FeO)/C mass ratio of between 2 and 3.2.
It is known that the coal used in the briquettes and pellets described above serve as a binder and a reductant. One disadvantage of using coal In agglomerates can be a decrease in strength if a coal with a high volatile content is used.
The present inventors have now developed an economically viable process for the agglomeration of titania slag particles smaller than 106 μm thereby rendering it suitable to be used in the chloride process. It has been found that the carbon content in the agglomerates can be reduced and still provide agglomerates with a suitable strength.